Marine grade plywood? I have such a deck on my solo pontoon. If the webbed decking frame is solid enough to support it you might find an aluminum piece of diamond plate. This may require some sort of brace under the decking because it is not tremendous for weight bearing. If you are not going to put a ton of stuff back there either should work. There are a few threads on this or similar pontoon modifications here. Some have some pretty cool photos. (bunch of boat geeks and all)
I used aluminum diamond plate for my Osprey rear deck 24"x60". I had two pieces of 1" square steel welded to the frame and attached the diamond plate with screws to the 1" steel. If I were to do it again I would cut a few slits in the diamond plate and use cam straps to attach it to the frame, it would be lighter without 2x60" pieces of steel and I would be able to take it off when not needed.
I have the same Bucks boat and I built a custom frame extension with some stainless tubing and some diamond plate. I used an electrical conduit bender to bend the tubing and a rivet gun to rivet the diamond plate to it. The extension slides right into the existing frame and clips on with clevis-style clips. It works awesome...I use it it put large totes, dry bags, etc for longer trips. It would be easy to mount a motor or anchor to it as well. PM me if you want more pics or details.
Bucks, I was facing a simliar situation in that I wanted to build a bigger deck, there's a previous post with what I ended up doing and what others did as well. Mine ended up working out nicely, easily carrying what I needed for an extended trip.